[11.01] Radiative Effects of CO2 in the Martian Polar Caps from MGS

G. B. Hansen (HIGP/SOEST, Univ. Hawaii)

We have examined data from two orbits in the assessment
period of the Mars Global Surveyor (MGS), focusing on the
known properties of pure CO2 ice. For CO2 snow
surfaces with grains less than a few centimeters in size,
there should be a depression in the thermal infrared
brightness temperature with a minimum near 25 \mum. These
grain-size properties of solid CO2 may explain both the
continuous and transient low-brightness-temperature regions
discovered by the Viking orbiter infrared radiometers
(IRTM). Using the Thermal Emission Spectrometer (TES) data
from Orbit 21, we show how the spectra from an annulus near
the edge of the retreating south polar cap are consistent
with smaller CO2 grain sizes. We suspect that this
behavior results from the fracturing of nearly solid sheets
of frozen CO2 (formed in this way, or reaching this state
by a process of grain-coarsening), as they are heated
(mostly from below) by the rising sun. The unfractured
layers near the pole behave more like a blackbody, and
probably reflect visible light much like the surfaces
underneath them, while the cracked surfaces should appear
bright. We also investigated a low-altitude pass over the
autumn north polar region in orbit 35, using data from both
TES and the Mars Orbiter Laser Altimeter (MOLA). We found an
area with properties similar to the Viking transient
low-brightness-temperature regions, located precisely on the
rim of a 22-km crater (from the MOLA profile) at the outlet
of the Chasma Boreale. The MOLA reflectivity and the
30-\mum emissivity are anticorrelated across this feature
(bright at 1 \mum, low emissivity at 30 \mum), implying
that it is caused primarily by the CO2 particle size. We
propose that this region is characterized by a local change
in particle size, and that it is correlated with topography,
a fact hinted at but not proven by Viking IRTM studies. A
likely cause of such a feature is the flow over the obstacle
by a possibly dusty atmosphere near saturation, causing a
local precipitation of CO2 snow nucleated on the dust
grains. The spectra of both south and north polar deposits
indicate varying amounts of dust contamination, with the
north spot having much more dust than the south.